The fuel injection valve according to the present invention, when viewed downstream to approximately the valve seat, is similar to conventionally configured injection valves, for example, dispenser injection valves as described in German patent application no. 35 33 521.
Proceeding therefrom, on the other hand, in functional terms, i.e. with reference to fuel delivery, the electromagnetic fuel injection valve according to the present invention has only a superficial similarity to a swirl valve as known, for example, from European Patent Application No. EP-OS 0 057 407, in that the fuel delivered by the fuel injection valve according to the present invention leaves the valve in the form of a conical lamella. Precisely in this respect, however, there are substantial differences between the present invention and known swirl valves in terms of both configuration and operation, which become evident especially in regard to a crucially finer droplet diameter. More detailed discussion below will therefore be devoted in particular to the operation of such known swirl valves, in which by means of a swirl extension--often arranged upstream from the valve needle seat--a swirl is imparted to the fuel escaping from the metering orifice so that it ultimately breaks apart into a conical lamella.
Also known in addition to the aforesaid dispenser injection valves and swirl valves are so-called apertured-spray valves, as described in German No. DE-OS 40 26 721, as well as impact valves, as described in U.S. Pat. No. 4,982,716.
All of these valves provide better fuel conditioning characteristics than the usual single-orifice valves. In the apertured-spray valves, including cap valves, in which fuel is metered through fixed aperture plates, an orifice plate that is usually deformed into a spherical shape is present so as to optimize fuel inflow in terms of spray angle and other factors. If aperture plates are present, they are usually implemented by means of oblique orifices, as is clearly evident in German patent application no. DE-OS 40 26 721 in the orifice plate located downstream from the valve ball.
However, infeed to the injection holes becomes asymmetrical if even the slightest turbulence occurs, so that the spray angles are dynamically so dissimilar because of their long jet length and the predefined small emergence angle until preparation, that they collide with one another and atomization is lost. The number of holes cannot be made as great as required for extremely fine conditioning.
In swirl valves, structural problems that cannot be remedied by fine-tuning are evident particularly in the fact that the diameter of the spray-off edge is very small as compared to the lamella thickness. High outlet turbulence thus results, which causes detrimentally fluctuating lamella length and is further aggravated by subsidiary eddies.
There can also be delays in the creation of the actual swirl, i.e. such valves react with a dynamic lag, for example, initially forming a straight stream. Since it is theoretically based on the cyclone principle, friction is inherently relatively high. Because of the small proportion of surface energy in the lamella in a swirl valve, the conical angle cannot be made as great as would be desirable for optimum conditioning.
Since, however, the present invention also aims for and achieves a particularly laminar form of hollow conical lamella of injected fuel, it should also be noted, for purposes of comparison with and differentiation from the behavior of swirl valves, that in the latter--quantitative values being provided for better comprehension--the conical angle of 90 degrees achieved for a small Sauter diameter (SMD) is favorable but functionally too large, since the required conical angle is .ltoreq.60 degrees for a single-point injection system and .ltoreq.25 degrees for a multipoint system.
In addition, the available fuel pressure is often low. When the valve is activated, the differential pressure and the associated velocity of the emerging fuel are so low that the aforesaid straight stream results because: the long fluid column of the swirl spiral must be accelerated; the opening cross section of the seat valve, which initially is only partly open, is less than that of the swirl device or the metering gap, so that initially the maximum fuel velocity, which is useless for conditioning, is present in the seat; and the pump volume must first be filled during the valve's opening stroke.
For the sake of completeness, it should also be mentioned that in impact valves, for example in U.S. Pat. No. 4,982,716, the fuel stream is directed onto an obstacle by which it can be deformed, for example into a turbulent conical lamella or into fan streams. Two streams can also be directed against one another in such impact valves.
If conditioning of the fuel in such valves is improved by air injection, then although the resulting Sauter diameter can be approximately halved, the droplet velocity is nevertheless typically tripled, which counteracts the desired final result of fine fuel misting with low droplet velocity.